Fatty substance esters are currently used in many applications as diesel fuels, furnace fuel oils, ecological solvents, base compounds for manufacturing fatty alcohol sulfonates, amides, ester dimers, etc.
In the case of diesel fuel, which is today a major application for fatty substance esters, a certain number of specifications have been established, whose list, limits and methods belong to standard EN 14,214 (2003) currently applicable in Europe. The ester must contain at least 96.5 mass % esters, at most 0.8 mass % monoglycerides, at most 0.2 mass % diglycerides and at most 0.2 mass % triglycerides, few free fatty acids (<0.5 mg KOH per g) that may be corrosive, less than 0.25 mass % bonded and free glycerin, and metals only as traces. This involves a precise protocol to obtain the desired purity.
When preparing an ester from oil or fat and monoalcohol, depending on the nature of the oil initially used, 10 to 15 mass % of a secondary product, which is glycerin, automatically forms. This glycerin can be valorized in many applications but it first has to be purified (removal of metals, salts, water). Vacuum bidistillation is often necessary in order to reach this purity.
In short, most commercial ester manufacturing methods lead quite readily to raw products (esters and glycerin) that however have to be deeply purified using various treatments that eventually burden the cost of the conversion.
It is well known to produce methyl esters using conventional means such as homogeneous catalysis with soluble catalysts, such as soda or sodium methylate, by reacting a neutral oil and an alcohol such as methanol (for example JAOCS 61, 343-348 (1984)). A pure product that can be used as fuel and a glycerin meeting the specifications are however obtained only after many stages. In fact, the glycerin obtained is polluted by alkaline salts or alcoholates, so that the glycerin purification plant is almost as costly as the ester manufacturing plant.
Heterogeneous catalysis methods afford the advantage of producing catalyst-free esters and glycerin, which are therefore easily purified. However, it is often difficult to economically obtain both an ester and a glycerin of high purity.
Many metal oxides have been used to catalyze the transesterification reaction. This was recently the case with lithium-doped (Xie et al., Ind. Eng. Chem. Res., 2007, 10.1021/ie070597s) or barium-doped zinc oxide (Xie et al., Catalysis Letters (2007) 117, 159-165). Reddy et al. (Energy Fuels, 2006, 20, 1310) suggest using nanocrystalline calcium oxide that thanks to the formation, in the presence of methanol, of Ca(OMe)2 species exhibits the behaviour of an essentially heterogeneous catalyst. Many authors have also studied the behaviour of magnesium oxide (Dossin et al., Applied Catalysis B, 2006, 61, 35-45). These alkaline-earth metal oxides have non-zero solubilities in methanol (Gryglewicz, Bioresour. Technol., 1999, 70, 249), which poses catalyst leaching and stability problems leading to a significant activity decrease upon recycling. The solution provided for zinc oxide based catalysts consisting in regenerating the catalyst by lithium or barium nitrate impregnation cannot be transposed to an industrial use. Furthermore, with this type of catalysts, the leached metal species are found in the products obtained (ester and glycerin), leading to a degradation of their quality and their non-conformance to the specifications imposed on biodiesel fuels. European patent EP-B-0,198,243 describes the manufacture of methyl esters by transesterification of an oil with methanol, using as the catalyst an alumina or a mixture of alumina and of ferrous oxide. However, the liquid hourly space velocity (volume of oil injected/volume of catalyst/hour) is low, the amount of glycerin collected is much less than that theoretically expected and the purity of the esters obtained is rather low (ranging between 93.5% and 98%).
Methods using a catalytic system based on metallic oxides, alone or in combination, deposited or not on an alumina, have been described. Patent FR-B-2,752,242 filed by the applicant describes the use of solid and non soluble catalysts formed from alumina and zinc oxide or zinc aluminate, wherein the free zinc oxide content is limited to 2 mass %.
Patent application EP-1,468,734 filed in the name of the applicant provides a method of preparing a catalyst comprising zinc aluminate, zinc oxide and alumina, and having mechanical crush resistance properties improved through substitution of zinc carbonate or nitrate for part of the zinc oxide in the preparation process. The free zinc oxide content is limited to 2 mass %. No catalytic test result is shown in this document.
Surprisingly enough, inventors have discovered that using a catalyst prepared according to a particular method of operation and containing more than 7 mass % free ZnO is not detrimental to the catalyst stability or leaching of the zinc in the reaction medium. On the contrary, it appears that it is advantageous to use these catalysts containing notably more than 2 mass % free ZnO, which is against the teaching of patent application EP-1,468,734, and having mechanical properties suited to use in industrial reactors.